1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine and the like for performing image formation in accordance with an electrophotographic method or an electrostatic recording method.
2. Related Background Art
As an electrophotographic multi-color or full-color image forming apparatus, there is proposed a so-called tandem type image forming apparatus which forms a color image by arranging a plurality of photosensitive drums in one row and sequentially superimposing a toner image of each color formed on each photosensitive drum onto an intermediate transfer member (intermediate transfer belt) or onto a transfer material.
FIG. 11 is a schematic block diagram showing an example of a conventional electrophotographic tandem type full-color image forming apparatus.
This image forming apparatus is provided with four image forming stations (image forming units), i.e., an image forming station 1Y for forming an image of a yellow color, an image forming station 1M for forming an image of a magenta color, an image forming station 1C for forming an image of a cyan color, and an image forming station 1K for forming an image of a black color, and these four image forming stations are arranged in one row at predetermined intervals.
In each of the image forming stations 1Y, 1M, 1C and 1K, photosensitive drums 2a, 2b, 2c and 2d are provided as image bearing members. Around the respective photosensitive drums 2a, 2b, 2c and 2d are provided charging rollers 3a, 3b, 3c and 3d, developing devices 4a, 4b, 4c and 4d, primary transfer rollers 5a, 5b, 5c and 5d, and drum cleaning devices (cleaning blades) 6a, 6b, 6c and 6d. Further, exposing devices 7a, 7b, 7c and 7d are respectively provided above portions between the charging rollers 3a, 3b, 3c and 3d and the developing devices 4a, 4b, 4c and 4d. Yellow toner, magenta toner, cyan toner and black toner are respectively contained in the respective developing devices 4a, 4b, 4c and 4d. 
In this conventional example, each of the photosensitive drums 2a, 2b, 2c and 2d is a negative charge organic photoconductor and has a photoconductive layer (not shown) on a drum base body (not shown) made of, e.g., aluminum. Each photosensitive drum is driven to rotate at a predetermined process speed by a drive device (not shown).
The charging rollers 3a, 3b, 3c and 3d (charging means) are respectively brought into contact with the photosensitive drums 2a, 2b, 2c and 2d by a predetermined pressure force and uniformly charge the surfaces of the respective photosensitive drums 2a, 2b, 2c and 2d by charging bias applied from a charging bias power supply (not shown) so as to provide a predetermined polarity and potential.
The developing devices 4a, 4b, 4c and 4d cause the toner to adhere to electrostatic latent images formed on the respective photosensitive drums 2a, 2b, 2c and 2d and develop (realize visible images) them as toner images.
Each of the primary transfer rollers 5a, 5b, 5c and 5d (primary transferring means) is constituted by coating a core with an elastic member having a medium-resistance (volume resistivity: 1xc3x97104 to 1xc3x97107xcexa9cm), and it comes into contact with each of the photosensitive drums 2a, 2b, 2c and 2d through the intermediate transfer belt 8 (an intermediate transfer member) at each primary transfer position. Primary transfer bias power supplies (high-voltage power supplies) 9a, 9b, 9c and 9d are respectively connected to the primary transfer rollers 5a, 5b, 5c and 5d. 
The exposing devices (laser scanner devices) 7a, 7b, 7c and 7d form electrostatic latent images according to image information on the surfaces of the respective photosensitive drums 2a, 2b, 2c and 2d charged by the respective charging rollers 3a, 3b, 3c and 3d by outputting from a laser output section (not shown) laser beams modulated in accordance with time series electric digital pixel signals of image information respectively input from a host computer (not shown) and image-exposing the surfaces of the respective photosensitive drums 2a, 2b, 2c and 2d through reflection mirrors 10a, 10b, 10c and 10d. 
The intermediate transfer belt 8 is stretched around a drive roller 11, a secondary transfer opposite roller 12 and a support roller 13 and rotated (moved) in a direction indicated by an arrow (clockwise direction) by drive of the drive roller 11. The secondary transfer opposite roller 12 is brought into contact with a secondary transfer roller 14 through the intermediate transfer belt 8 to form a secondary transfer position M. As the intermediate transfer belt 8, one having a volume resistivity of 1xc3x97108 to 1xc3x971012xcexa9cm is preferable, and one consisting of urethane based resin, fluorine-based resin, nylon-based resin or polyimide-based resin, one consisting of an elastic material such as silicone rubber or Hydrin rubber, or one obtained by dispersing carbon or conductive powder in these materials and performing the resistivity adjustment can be used. In this conventional example, a single-layer endless belt having a thickness of 0.1 mm obtained by dispersing carbon in polyimide and adjusting the volume resistivity to 1xc3x97109xcexa9cm was used as the intermediate transfer belt 8.
The secondary transfer roller 14 is constituted by coating a core with a foam layer of, e.g., EPDM, NBR, Hydrin rubber having a medium-resistance value (volume resistivity: 1xc3x97106 to 1xc3x9710xcexa9cm) and provided at a position opposed to the secondary transfer opposite roller 12 with the intermediate transfer belt 8 and a transfer material P such as paper sandwiched therebetween so as to be capable of contacting with and being separated from them. Further, to the secondary transfer roller 14 are connected a current detector 17 for applying and controlling the later-described secondary transfer bias, a power supply 18, and a controller 19.
The belt cleaning device (cleaning blade) 15 for removing and collecting transfer residual toner remaining on the surface of the intermediate transfer belt 8 is provided at the outside of the intermediate transfer belt 8. Further, a fixing device 16 having a fixing roller 16a and a pressure roller 16b is provided downstream of a secondary transfer position M in the direction for conveying the transfer material P where the secondary transfer opposite roller 12 and the secondary transfer roller 14 come into contact with each other.
The image forming operation by the above-described image forming apparatus will now be described.
When an image forming operation start signal is sent, transfer material (paper) P is fed one by one from a cassette (not shown) and conveyed to a registration roller (not shown). At this time, the registration roller (not shown) is stopped and the leading end of the transfer material P is in the standby mode immediately before the secondary transfer position M.
On the other hand, in the respective image forming stations 1Y, 1M, 1C and 1K, when the image forming operation start signal is sent, the respective photosensitive drums 2a, 2b, 2c and 2d driven to rotate at a predetermined process speed (0.117 in/see) are uniformly charged so as to have a negative polarity by the charging rollers 3a, 3b, 3c and 3d, respectively. The exposing devices 7a, 7b, 7c and 7d respectively convert color-separated image signals input from a host computer (not shown) into optical signals in the laser output section (not shown), and the modulated laser beams (the converted optical signals) are respectively scan-exposed through the reflection mirrors 10a, 10b, 10c and 10d onto the respective charged photosensitive drums 2a, 2b, 2c and 2d so as to form electrostatic latent images thereon.
The intensity and the irradiation spot diameter of each laser beam are appropriately set by using a resolution of the image forming apparatus and a desired image density. The electrostatic latent images on the respective photosensitive drums 2a, 2b, 2c and 2d are formed when a portion irradiated with the laser beam is held at a light section potential (xe2x88x92150 V) and a portion irradiated with no laser beam is held at a dark section potential (xe2x88x92550 V) charged by the respective charging rollers 3a, 3b, 3c and 3d. 
At first, the developing device 4a to which developing bias (in this conventional example, bias obtained by superimposing an altenating component (rectangular wave having a peak-to-peak voltage of 1.5 kV and a frequency of 3 kHz on a direct current component (xe2x88x92400 V)) having a polarity equal to the charging polarity (negative polarity) of the photosensitive drum 2a causes yellow toner to adhere to the electrostatic latent image formed on the photosensitive drum 2a, thereby visualizing it as a toner image. This yellow toner image is primary transferred onto the rotating intermediate transfer belt 8 by the primary transfer roller 5a to which the primary transfer bias (having a polarity (positive polarity) opposite to that of the toner) from the primary transfer bias power supply 9a. 
The intermediate transfer belt 8 onto which the yellow toner image is transferred is moved to the image forming station 1M side. In the image forming station 1M, a toner image of magenta similarly formed on the photosensitive drum 2b is superimposed onto the toner image of yellow on the intermediate transfer belt 8 and transferred by the primary transfer roller 5b to which the primary transfer bias (bias having a polarity opposite to that of the toner (in this conventional example, +300 V)) from the primary transfer bias power supply 9b. 
Subsequently, toner images of cyan and black formed on the photosensitive drums 2c and 2d in the image forming stations 1C and 1K are similarly sequentially superimposed on the toner images of yellow and magenta, which are superimposed and transferred onto the intermediate transfer belt 8, by the primary transfer rollers 5c and 5d to which a primary transfer bias (bias having a polarity opposite to that of the toner (in this conventional example, +300 V)) is applied from the respective primary transfer bias power supplies 9c and 9d. 
In accordance with the timing at which the full color toner image leading end on the intermediate transfer belt 8 is moved to the secondary transfer position M, the transfer material P is conveyed to the secondary transfer position M by the registration roller (not shown), and the full-color toner image is collectively secondary-transferred onto the transfer material P by the secondary transfer roller 14 on which the secondary transfer bias (bias having a polarity opposite to that of the toner (in this conventional example, +20xcexcA)) is applied.
The transfer material P having the full-color toner image formed thereon is conveyed to the fixing device 16, and the full-color toner image is heated and pressed by a fixing nip between the fixing roller 16a and the pressure roller 16b so as to be thermally fixed on the transfer material P. Thereafter, the transfer material P is delivered to the outside, thereby terminating a series of the image forming steps.
It is to be noted that the primary transfer residual toner remaining on the photosensitive drums 2a, 2b, 2c and 2d is removed and collected by the drum cleaning devices 6a, 6b, 6c and 6d at the time of the above-described each primary transfer. Further, the secondary transfer residual toner remaining on the intermediate transfer belt 8 after the secondary transfer is removed and collected by the belt cleaning device 15.
Description will now be given as to the secondary transfer bias control with respect to the secondary transfer roller 14 in this conventional example.
In this conventional example, the electric current detector 17 detects an electric current which flows when a voltage is applied from the power supply 18 to the secondary transfer roller 14 by control of the controller 19. Based on a detection signal input from the current detector 17, the controller 19 controls the transfer electric current by controlling the output voltage of the power supply 18 in such a manner that the detection signal has a predetermined value.
The control of the transfer electric current will now be described with reference to a flowchart shown in FIG. 12.
When a voltage V is applied to the secondary transfer roller 14 upon starting the constant current control (step S1), the electric current detector 17 detects a value of an electric current which flows in response to application of the voltage V, and this value is converted into an analog signal. After being input to the controller 19, this analog signal is subjected to A/D conversion so as to be converted into a detection electric current value i which is a digital value (step S2).
The detection electric current value i obtained in the step S2 is compared with a target electric current value i0 previously stored in the controller 19 (step S3). If a difference xcex94i (=i0xe2x88x92i) between these values is larger than 0 (0 less than xcex94i), a voltage value (step S4) obtained by adding a predetermined voltage variable quantity xcex94V to the output voltage V is set. Further, if the difference xcex94i (=i0xe2x88x92i) is 0 (0=xcex94i), the output voltage V at that time is held (step S5). Furthermore, if the difference xcex94i (=i0xe2x88x92i) is smaller than 0 (0 greater than xcex94i), a voltage value obtained by subtracting a predetermined voltage variable quantity xcex94V from the output voltage V is set (step S6).
After making judgment upon whether the above-described constant electric current control is continued or terminated (step S7), if this control is continued, the set voltage value V is applied (step S8), and the processing then returns to the step S2. By repeating the above-described operation, the transfer electric current is converged to a target electric current value, and the electric current control is realized in the power supply 18. As to judgment upon whether the control is continued or terminated (step S7), the control may be terminated upon completion of the transfer operation involved by the image formation, for example. It is to be noted that the target electric current value i0 is +20 xcexcA and the voltage variable quantity xcex94V is +20V in this conventional example.
For the meantime, in the above-described image forming apparatus using the constant electric current control, it is known that the following problems occur in the above-mentioned secondary transfer step.
Impedance of a transfer system (path along which the transfer electric current flows from the power supply through the transferring means) varies due to a change in the environment (temperature or humidity) in which the image forming apparatus is provided, a type of the transfer material P, an individual difference of the secondary transfer roller 14 and the like as the transfer member. As a result, in a low-temperature and low-humidity environment LL (for example, a temperature: 15xc2x0 C. and a humidity: 10%), rising of the power supply 18 becomes slower than that in the ordinary-temperature and ordinary-humidity environment JJ (for example, a temperature of 23xc2x0 C., a humidity: 60%) because of increase in the resistance of the secondary transfer roller 14. Consequently, convergence to the target electric current value i0 takes time when a sudden fluctuation in load occurs, thereby generating a failure in an image such as a light image due to defective transfer in a leading end area of the transfer material P and the like.
On the other hand, in the high-temperature and high-humidity environment HH (for example, a temperature: 30xc2x0 C. and a humidity: 80%), rising of the power supply 18 becomes faster than that in the ordinary-temperature and ordinary-humidity environment (for example, a temperature: 23xc2x0 C. and a humidity: 60%) because of decrease in the resistance of the secondary transfer roller 14. As a result, overshooting or undershooting of the transfer bias occurs when the load suddenly fluctuates, thereby generating a failure of an image such as a light image due to defective transfer in a leading end area of the transfer material P and the like.
It is, therefore, an object of the present invention to provide an image forming apparatus capable of preventing a failure of an image involved by a change in impedance of a transfer system due to a change in the environment (temperature, humidity), a type of a transfer material, or an individual difference of a transfer member (such as a transfer roller).
To achieve this aim, according to the present invention, there is provided an image forming apparatus capable of changing a predetermined voltage quantity comprising:
an image bearing member for bearing a developer image;
transferring means for electrostatically transferring the developer image onto a transfer material;
a power supply for applying a voltage to the transferring means;
electric current detecting means for detecting a value of an electric current flowing to the transferring means when a voltage is applied from the power supply to the transferring means; and
controlling means for controlling an output of the power supply by varying a voltage applied to the transferring means in such a manner that the electric current value detected by the electric current detecting means becomes a predetermined target electric current value.